Generic applicability of cyclodextrins in chromatographic separation and purification processes is described at length in reviews by W. L. Hinze, Separation and purification methods, 1981, 10(2), 159–237. Y. Kawaguchi, et al., Anal. Chem., 1983, 55, 1852; D. W. Armstrong, et al., Anal. Chem., 1985, 57, 234 and S. Li, et al., Chem. Rev., 1992, 92, 1457. Chromatographic separation on chiral stationary phases (CSP) is also the most convenient analytical method for the determination of enantiomeric purity (see for example S. G. Allenmark, Chromatographic Enantioseparations: Methods and Applications, 2nd ed., Prentice Hall, N.J. , 1991). In recent years, tremendous research efforts were made in bonding cyclodextrins to solid matrices, such as silica gel, via amino or amido linkages. However, these bonds are inherently unstable to hydrolysis, thus placing severe limitations on use of these materials in aqueous media. Alternative approaches for immobilizing cyclodextrin using hydrolytically more stable ether linkages (U.S. Pat. No. 4,539,399) or carbamic acid moieties (U.S. Pat. No. 5,104,547) were also investigated. However, in all these approaches, regioselective derivatisation of cyclodextrin cannot be readily effected due to the presence of multiple hydroxy moieties in the cyclodextrin starting materials. Thus, reaction may take place on the 2, 3 or 6-position of glucose moieties of cyclodextrin, which is hard to resolve.
It has been reported that derivatised cyclodextrin stationary phases for liquid chromatography show definite enantioselectivity for a variety of compounds while pristine cyclodextrin bonded stationary phases display low enantioselectivity. Enantioselectivity of the materials was generally improved by increasing the degree of derivatisation of the —OH groups on cyclodextrin with carbamate groups, and by increasing the surface concentration of cyclodextrin immobilized on the support materials (D. W. Armstrong et al., Anal. Chem., 1990, 62, 1610; T. Hargitai et al., J. Chromatogr., 1993, 628, 11; T. Hargitai, et al., J. Liq. Chromatogr., 1993, 16(4), 843). In order to maximize the extent of cyclodextrin derivatisation, large molar excesses of derivatising reagents under vigorous conditions were often used. However, the derivatisation processes invariably involved the prior immobilisation of underivatised cyclodextrin on the support material followed by derivatisation procedures involving solid-liquid phases. This usually results in partial derivatisation of the hydroxyl groups of the cyclodextrin and also in large, sterically encumbered cyclodextrins having a low extent of derivatisation. These methods did not give good reproducibility or uniformity of product, with the consequence that separation of enantiomers may vary from batch to batch of the obtained CD-based CSP.
Ng, et al., U.S. Pat. No. 6,017,458 describe a procedure of immobilizing perfunctionalized cyclodextrin onto the surface of a support. The patent says that the cyclodextrin is immobilized via a urethane linkage, but it is believed that this is not correct and the linkage is a urea linkage. The procedure provides an efficient method with well-defined chemical structure and very good reproducibility. However, in these examples, and in the other instances listed above, the cyclodextrins were immobilized onto the support as small molecules, which potentially limit their stability in mobile phases with high aqueous content. Although the patent mentions monoazido and diazido cyclodextrins, only monoazido cyclodextrins are used in the examples of the patent and each immobilized cyclodextrin will have only one urea linkage linking it to the support material.
Polysiloxane with cyclodextrin anchored to its side chain has been prepared and coated onto the surface of silica gel. This material exhibits interesting properties in reverse phase HPLC. (V. Schurig, et al., J. Chromatogr. A, 1996, 755, 299; V. Schurig, et al., Ger Offen DE 43 24 636 A1 (1994), V. Schurig, et al., Angew. Chem. Int. Ed. Engl., 1994, 33, 2222). However, there is no report of the cyclodextrin polymer immobilized onto a support and applied in chiral separation.